1. Field of the Invention
The invention pertains to the field of optoelectronic devices. More particularly, the invention pertains to semiconductor light emitting devices for visible and infra-red spectral ranges.
2. Description of Related Art
There is a need in the light sources in the visible, particularly, in green-yellow and bright-red spectral ranges. Furthermore, there is a continuous need to improve temperature stability of the characteristics of the devices emitting in the near infrared spectral range. An improvement in the temperature stability of the characteristics of far-infrared light-emitters based on intrasubband transitions such as cascade lasers is also demanded. The limitations of the traditional heterostructure lasers and light emitting diodes are related to the fact that the spectral range and/or the temperature stability of the characteristics of the devices are defined by the maximum value of the forbidden gap, which can be realized for the direct bandgap materials, on the one hand, and by the availability of potential barriers preventing thermal escape of injected nonequilibrium carriers from the active region, on the other hand. Presently the problem has no universal solution, and each spectral range is covered by the preferred materials system. For example, bright red lasers and light-emitting diodes are produced using InyGa1-x-yAlxP materials and the related heterostructures deposited on GaAs substrates. Green and blue light emitting diodes (LEDs) are produced using InxGa1-xN or InyGa1-x-yAlxN materials on sapphire, GaN, silicon or silicon carbide substrates. Infrared light emitters at wavelengths longer than 800 nm are typically produced using the AlxGa1-xAs materials system on GaAs substrates. InyGa1-x-yAlxAs or InxGa1-xAs1-yPy materials on InP substrates are preferred for light emitters at 1300 nm and longer wavelengths. Some extension of the wavelength range towards longer wavelength on a particular substrate can be additionally made by employing thin elastically strained insertions of narrow bandgap material in the form of layers, quantum wells (QWs), quantum wires (QWWs) or quantum dots (QDs). However, an opportunity to shift the emission spectrum towards the shorter wavelength using a similar approach was not demonstrated so far. For example, despite the fact that the direct bandgap of AlAs is ˜3.5 eV corresponding to ˜350 nm wavelength matching the ultraviolet range, it is not possible to realize efficient light sources in this material due to the indirect bandgap nature of this binary material. The use of AlxGa1-xAs wells or GaAs—AlAs superlattices having a direct bandgap structure to generate an emission spectrum is restricted to ˜2 eV or 620 nm (bright red) or smaller energy and longer corresponding wavelength and even this wavelength cannot generally be reached at high efficiency due to the lack of confinement of nonequilibrium carriers in the light emitting device made of AlxGa1-xAs materials. In the conventional approach, for example, making monolithic white light emitters is generally not possible if only one materials system is used.
The solution to achieve white light emission in light-emitting diodes (LEDs) is presently based either on down conversion of the blue light using phosphorus or by heterogeneous integration of different light sources produced in different materials systems with the spectral mixing of the generated light. Lasers designed for different spectral ranges are typically produced using different materials. The temperature stability of the performance of the devices is typically sacrificed as the choice of the materials to achieve the necessary electron and hole confinement energies is limited.
Aluminum gallium arsenide (AlxGa1-xAs) is presently broadly used in micro- and optoelectronics. In the whole range of aluminum compositions the lattice parameter of the material is closely lattice matched to the GaAs substrate and the strain is minimum even in the case of rather thick layers. Moderate concentrations of In, P, Sb or other atoms can be introduced in the material fractionally replacing by substitution some of the group III (Ga, Al) or the group V (As) atoms, respectively, to form strained insertions (quantum wells, quantum wires, or quantum dots). InxGa1-xAs insertions are typically introduced to adjust the wavelength of the gain material beyond the spectral range covered by GaAs and to reach wavelengths longer than ˜870 nm, while GaAs1-yPy layers, which have a larger bandgap as compared to GaAs are typically used in light-emitting devices for strain compensation of indium- or antimony-containing materials. The InyGa1-x-yAlyP materials system is suitable for bright-red LEDs and laser diodes. However, the lowest bandgap phosphide binary material in this system InP has a bandgap of 1.34 eV.
Furthermore, due to a significant ˜3.8% lattice mismatch three-dimensional islands are usually formed after the deposition of only a few (2-3) monolayers of InP on the InxGa1-xP surface, all on a GaAs substrate. Photoluminescence bands within the 750-680 nm spectral range are typically observed from these predominantly biaxially compressively strained QD insertions. These structures are hardly suitable for the majority of infrared devices emitting at wavelengths longer than 800 nm.
Thus, there exists a need to extend the spectral range of the AlxGa1-xAs-based light emitting devices on GaAs substrates from infrared and deep red towards bright red, orange, yellow or green. On the other side, there is a need to increase the temperature stability of the characteristics of the AlGaAs-based devices operating in the conventional infrared spectral range.